This invention relates generally to material surveillance method and apparatus. More particularly, it concerns the use of a serially energized, collimated microwave transmitter/receiver array to interrogate a region through which articles are conveyed to create an imageable, three-dimensional material nature and shape profile of real-time dielectric-constant conditions therein. The method and apparatus of the invention are described in the context of airport luggage surveillance systems for use in the detection of contraband or hazardous conditions.
Growing concern over airport and airline flight security against bomb threats and drug smuggling requires a diligent effort at improving the ability to screen luggage and interdict contraband. Traditionally, X-ray equipment has been used to provide a two-dimensional snapshot of the X-ray energy opacity of materials. It is capable of crudely discriminating between low- and high-density materials, i.e. it is useful in detecting high-metal-content items including most firearms. Three-dimensional X-ray screening equipment is not widely used, as it is prohibitively slow and costly, and requires multiple scanner arrays, mechanical scanning movement and substantial power. The high energy levels required for high-resolution X-ray scanning and the adverse, cumulative, physiological effects even of low-level X-radiation on living tissue require inordinate protective shielding measures. Even with proper shielding, there remains a significant health risk to operating personnel and to the public.
Ultrasound has found application in the medical arts, for fluid and soft body tissue boundary measurement and imaging. But ultrasound detection of contraband is impractical because the target materials are poor sonic media and because coupling ultrasonic energy to such materials is virtually impossible. Further, ultrasound lends itself better to sensing ultrasonic permeability changes in homogeneous materials, or boundaries between heterogeneous materials, than to classifying or identifying constituent materials.
Thermal neutron analysis represents a most recent advance in luggage scanning techniques. The technique depends upon bombardment of luggage with low-energy neutrons. The neutrons cause nitrogen, which is a constituent of most explosives, to emit gamma radiation that can be detected and analyzed by computer. It is reported that such equipment tends to render false-positive indications of explosives, due to commonly occurring high-nitrogen concentrations in wool, leather, plastics and foodstuffs. Thus, while this technique is sufficiently sensitive to detect, for example, plastic explosives, it is insufficiently selective to efficiently and accurately screen luggage. Of course, it is also extremely slow (requiring many seconds to screen each article), unsafe at any speed and limited to detection of materials that exhibit high nitrogen concentrations, which limitation renders it incapable of screening luggage for a wide variety of contraband material characteristics. Finally, because it is a new technology, thermal neutron analysis is very expensive, with present costs estimated to be an order of magnitude higher than conventional X-ray luggage screening.
As disclosed in my U.S. Pat. No. 4,234,844, issued Nov. 18, 1980, entitled "Electromagnetic Noncontacting Measuring Apparatus", it is possible to monitor electrical parameters by generating and focusing electromagnetic energy at a pair of electrically related, spaced focal points. According to the teachings of that patent, a bidirectionally focusing microwave antenna (as described in my companion U.S. Pat. No. 4,318,108, issued Mar. 2, 1982) can be used to interrogate living tissue by positioning one of its focal points adjacent an interrogation zone in a low-powered, non-contacting arrangement. By utilizing a receiver located adjacent the other focal point, it is possible, by monitoring voltage, current and phase conditions at the receiver, to determine related electrical conditions within the interrogation zone. One such electrical condition that has been found useful in medical monitoring and diagnosis is the dielectric constant of materials within the zone, as is described in my copending U.S. patent application Ser. No. 06/693,388, filed Jan. 22, 1985, entitled "Dielectric-Constant Change Monitoring", which issued as U.S. Pat. No. 4,947,848 on Aug. 14, 1990. These patent and patent application disclosures are incorporated herein by this reference.
It is a principal object of the invention to provide improved security/surveillance method and apparatus for use in luggage screening for contraband and/or hazardous condition detection.
Another object is to provide such method and apparatus that is capable of real-time, three-dimensional profiling and imaging of a wide variety of material contents of luggage.
It is still another object to provide such method and apparatus that uses a non-contacting, collimated array of bidirectionally focusing transmitter/receiver antennae defining an interrogation region through which articles are conveyed.
Yet another object is to provide a method for serially timed dielectrometric scanning of the nature and shape of material moving through an interrogation region.
It is also an object to analyze the timed dielectric response of materials to microwave energy incident thereon by computer analysis to identify contraband that may be within such materials.
Another object is to provide such method and apparatus that accurately and efficiently can screen for a variety of material types and amounts, with fewer false-negative and false-positive contraband indications than is possible with existing technologies.
It is a further object of the invention to provide such security/surveillance method and apparatus that may be operated safely and inexpensively.
These and other objects and advantages of the invention will become more fully apparent by reference to the following description and the accompanying drawings.